A mortar grouting type joint for reinforcing bars consists of a hollow cylindrical body. Reinforcing bars are inserted into it through the openings at its both ends, and the cylindrical body is filled with mortar to thereby accomplish a butt type connection between a pair of reinforcing bars. This type of joint, which is disclosed in Japanese Patent No. 1848027, etc., is widely used in practice.
Conventionally, this joint has been used in vertical main reinforcing bar connection between PC members. That is, after placing a lower PC member from whose upper surface the upper end portions of main reinforcing bars protrude upwards, an upper PC member in whose lower end portion are embedded mortar grouting type joints for reinforcing bars with the lower end portions of main reinforcing bars inserted therein, is placed in such a manner that the above-described exposed reinforcing bars are accommodated in the embedded joints, and then the embedded joints are filled with mortar. This arrangement is advantageous in that the upper and lower PC members are connected to each other in a condition in which they are substantially in contact with each other, and that it is possible to connect a number of vertical reinforcing bars together simultaneously by a single process.
JP, A, 9-189097 discloses a mortar grouting type joint in which there is provided on the inner wall surface of a tubular sleeve a supporting protrusion perpendicularly to the central axis of: the sleeve and in which a reinforcing bar is pressed against the supporting protrusion by a bolt screwed in through a bolt hole provided in the portion of the inner wall surface of the tubular sleeve facing to this supporting protrusion, thereby securing the reinforcing bar in position.
Recently, large diameter reinforcing bars have come to be generally used for shorter term of works and labor saving. As a result, mortar grouting type joints for reinforcing bars as disclosed in Japanese Patent No. 1848027, etc. are often used for so-called bare connection of reinforcing bars and, in particular, the bare connection of pre-assembled reinforcing bar structures. This is due to the fact that, as the reinforcing bar diameter increases, the reinforcing bar connecting means conventionally used in this field (gas pressure welding, screw coupler system, etc.) deteriorate in reliability and lead to increased difficulty in operation, whereas the mortar grouting type joint for reinforcing bars involves no such problems even in the case of large diameter reinforcing bars.
In the case of the bare connection of pre-assembled reinforcing bar structures (which includes vertical connection and horizontal connection), there are a large number of pairs of reinforcing bars to be connected together. In the conventional connecting methods, the connection is impossible unless all of them are in a perfect coaxial relationship. Thus, after placing the reinforcing bar structure to be connected at a predetermined position with respect to the other reinforcing bar structure already installed, alignment has to be performed to effect coaxial adjustment for each of the large number of pairs of reinforcing bars, which involves a very difficult and time-consuming operation.
When effecting the connection by means of mortar grouting type joints, one end of each joint is attached to a reinforcing bar of the already installed reinforcing bar structure beforehand, and alignment is roughly performed between the reinforcing bars of the structure to be connected and those of the already installed structure. Then, the joints are pulled back to the side of the structure to be connected and the facing portions of the reinforcing bars of each pair are positioned at the center in the longitudinal direction (hereinafter simply referred to as the center) of the joint to thereby attain a substantially coaxial relationship. In this case, if a perfect coaxial relationship cannot be attained, connection is possible by filling the joints with mortar.
Thus, with the mortar grouting type joint, connection is possible even if the reinforcing bars are not in a perfect coaxial relationship, which means it is superior in operational efficiency.
This is due to the fact that there is a considerable margin between the inner diameter of the joint and the outer diameter of the reinforcing bars, and that deviation of the reinforcing bars can be offset to some degree by the joint. This margin has been very effective in the mode of use of the conventional mortar grouting type joint, which is embedded in precast concrete.
However, this disadvantageously involves an increase in the thickness of the joint. In the case of a joint for bare reinforcing bars, both the reinforcing bars and the joint are freely movable to some degree, so that this dimensional margin may be small, making it possible to reduce the size of the entire joint as compared with the conventional joints.
Further, the mortar with which the joint is filled takes a considerable amount of time before it cures. In the conventional method of using a mortar grouting type joint in which the joint is embedded in precast concrete, there is no need to secure the joint in position until the mortar cures, whereas, in the case of the connection of bare reinforcing bars, securing is necessary until the mortar cures.
Also, when the mortar grouting type joint for reinforcing bars having a supporting protrusion as disclosed in JP, A, 9-189097 is used for the connection of bare reinforcing bars, the following problem is involved: since the supporting protrusion is formed perpendicularly to the central axis of the sleeve, that is, formed along the inner periphery of the sleeve, the mortar flow along the sleeve central axis is hindered by the supporting protrusion when filling the sleeve with mortar, so that there is a large grouting resistance or space unfilled with mortar (void) is formed on the back side of the supporting protrusion, resulting in a deterioration in the reinforcing bar connecting performance of the joint.